Field of the Invention
The present invention relates to control of a machining path in a cutting process using a machine tool, and particularly to a machine tool controller capable of automatically generating, with simple operation, a cut-in machining path called a roll-in path and replacing a cut-portion machining path in a machining program with the roll-in path.
Description of the Related Art
When a machine tool is used to perform cutting in a direction perpendicular to a spindle (milling, see FIG. 8), such as grooving, a large amount of vibration occurs immediately after the cutting starts and undesirably lowers the life of a cutting tool.
A cutting tool 2 for milling is typically formed of a plurality of blades 3, as shown in FIG. 8. Observation of cutting action of one of the blades shows that rotation of the spindle (in a spindle rotating direction M) and movement Q in a direction of an axis perpendicular to the spindle cause the blade 3 to hit a workpiece 4 and produce cut chips.
The blade 3 of the cutting tool 2 hits the workpiece 4 and then goes out of the workpiece 4 because the spindle rotates, but the movement Q of the axis perpendicular to the spindle takes place simultaneously for milling. When the cutting tool 2 cuts into the workpiece 4 linearly, thick cut chips are produced when the cutting tool 2 goes out of the workpiece 4 until the cutting tool 2 cuts into the workpiece 4 by a distance corresponding to the radius of the cutting tool 2 (one-half the cutting tool diameter D). Since the cutting tool 2 produces thick cut chips immediately after the cutting tool 2 cuts into the workpiece 4, a large amount of vibration occurs when the cutting tool 2 goes out of the workpiece 4 and undesirably lowers the life of the cutting tool.
To solve the problem, there is a generally known method using a cut-in machining path called a roll-in path shown in FIG. 9. The roll-in path refers to a cutting tool path along which the cutting tool 2 cuts into the workpiece 4 in a milling process and enters the workpiece 4 while drawing an arc 8 in such a way that a cut chip exit position 7 is close to a location where an end surface 6 of the workpiece 4 intersects a machined groove width line 5 until the cutting tool 2 cuts into the workpiece 4 by a distance corresponding to the radius of the cutting tool 2 (one-half of the cutting tool diameter D). Using the roll-in cutting tool path allows the cutting tool 2 to cut in the workpiece 4 with cut chips maintained thin when the cutting tool 2 goes out of the workpiece 4. It can therefore be said that the roll-in cutting tool path is a machine-friendly machining path along which a small amount of load acts on the cutting tool 2 because the cutting tool 2 can cut into the workpiece 4 with cut chips maintained thin when the cutting tool 2 goes out of the workpiece 4.
However, calculating a machining start position where the roll-in path starts, for a machining program that has been inputted to a controller that controls a machine tool, or when the machining program is created, and reflecting an arcuate cut-in start path in the machining program is a cumbersome task. A machining method which uses a cut-in-portion machining path called a roll-in path has not become wide use in a market.
On the other hand, a technology for preventing breakage of a cutting tool in a machine tool is disclosed, for example, in Japanese Patent Application Laid-open No. 2004-252839. The technology involves detection of a load acting on the spindle that rotates a cutting tool or a load acting on a cutting tool feed shaft and prevention of breakage of the cutting tool in accordance with the magnitude of the detected load.